Many large Pacific Northwest public campgrounds offer evening entertainment in cozy amphitheater settings, usually on weekend evenings. Park rangers frequently talk about something of interest in the area, perhaps the life cycle of moths or mating habits of chipmunks.

Grand Coulee Dam is different. It eliminates the talking naturalist and ignores program variety. Instead, it repeats the same laser show every night of the summer camping season to the delight of thousands.

Grand Coulee Dam, which has handled as many as 70,000 visitors for one show, isn't exactly a cozy setting. Even though the nearest campground is three miles away, the laser light show serves the same function as amphitheater talks _ to provide campers with an informative and entertaining evening program.

After dinner, as the temperature begins to cool and dusk draws near, visitors to north-central Washington head for the dam. Instead of walking from their campsite to an amphitheater, they drive as much as 90 miles from Wenatchee and Spokane or from camps at Chelan, Sun Lakes and Lake Roosevelt.

''We have big crowds on most summer nights,'' said the operator of one of the laser shows. ''I've never heard anybody say they didn't enjoy the show.''

Only once during the laser show's first six years of existence did the crowd go home disappointed.

''We've had to cancel one performance,'' the operator said. ''We try our best to not let that happen because we know people come from a long way to see the show. We have redundancies on all our systems and can give a show with only two of the four lasers.''

The laser show cost the Bureau of Reclamation $785,700 to install in 1988. The spending was authorized by Congress because of its benefit as a free tourist attraction and to replace the worn-out colored lights that had lit Grand Couleee Dam for 30 years.

The largest of the Columbia River dams, Grand Coulee is still one of the engineering marvels of the world since its completion in 1941, then expansion in 1983. Portlandlers don't really know the Columbia River's entire story until they drive the 340 miles to see Grand Coulee Dam.

Only a few feet short of one mile wide, Grand Coulee Dam is the largest concrete structure and producer of electricity in the United States. The laser show has ample room to paint 200-foot eagles and one-third actual size aircraft carriers on the dam's 1,650-foot wide and 350-foot high spillway.

Operated out of the Visitor Arrival Center, the lasers shoot light up to 4,000 feet to the face of the dam. Comparatively, indoor laser shows at OMSI in Portland and Laserium in Seattle have much shorter targets.

The four lasers transmit light through a system of mirrors the size of a little fingernail. The single dot of light from each laser moves so quickly that the human eye sees solid figures moving across the dam. The show describes the geology and history of the Columbia River, as well as the hydroelectric, irrigation and recreation benefits of Grand Coulee Dam.

Lasers are used in medicine, the defense industry and entertainment. Although a laser beam is powerful enough to cut steel, the dam is not damaged because of the lasers' low intensity, the distance the beams travel and the diffusion by mirrors.

''If we had a dollar for all the people who have watched this over the years we'd be rich,'' the operator said.

''I wish I had a nickle,'' his side kick added.

The 28-minute show is offered for free, nightly at dusk, from the Saturday of Memorial Day weekend until Sept. 30.

When it's been a 100-degree day in Coulee country, the instant the spillway is opened for the show to be broadcast on falling water, it feels as though someone turned on the air conditioning.